If factory ammunition is the concern and you don't want to go with an outfit like Superior, why not just stick with .470? I know of two stores within an hour of me stock it (and I'm guessing one other but I won't give Griffin and Howe a penny), as well as Cabelas. I'm not disputing the need to reload mind you - I'm just saying factory ammo is easily available.

Alternatively you could go with a Blaser in .416 Rem Mag